这个国家正处在一场深刻社会变革的开端。

The country is ___on ___the ___threshold ___of a period of profound social change.

站在重大科学突破的门槛上，研究人员夜以继日地工作。

___On ___the ___threshold ___of a major scientific breakthrough, the researchers worked day and night.

站在新职业生涯的起点，她感到既兴奋又紧张。

Standing ___on ___the ___threshold ___of a new career, she felt both excited and nervous.